Diversification of angiosperms

What is the sister group to angiosperms?

Amborella trichopoda

What did the first angiosperm probably look like?

No proper xylem vessels, no separate petals or sepals, anthers produced a lot of pollen, carpels were not fused together
^This shows that angiosperms have since become more complex

How have angiosperms diversified so quickly?

They can manipulate animals to transfer its pollen to the carpel of another flower (to mix up the gametes)

Why is using animals better than using the wind^?

It is more precise and it doesn’t require a large, feathery stigma (which could pick up pollen from other species and clog the tubes)

What are pollination syndromes?

Flower traits that have evolved to attract different types of pollinators

What is beetle pollination?

Cantherophily e.g. in lillies

• These flowers provide extra pollen and nectar on flat surfaces because beetles have mouth parts positioned parallel to the axis of the body 

• Beetles don’t have good colour vision, but they rely heavily on scent

What is fly pollination?

Myophily e.g. Asteraceae (daisy family)

• Plants will flower in adverse conditions or odd times of the year because flies aren’t strictly periodic 

• Flies don’t feed their offspring so don’t require much food 

• Flies are more visual animals and have a preference for pale and yellow colours 

What is bee pollination?

Melittophily e.g. snapdragons

• Bees can perceive depth, so nectar is kept at the bottom of deep corolla tubes 

• Bes can see in ultraviolet, blue, yellow, but not red (so the flowers are usually brightly coloured)

What is butterfly pollination?

Psychophily e.g. Asteraceae (daisy family)

• Nectar is kept in deep corolla tubes 

• Butterflies have good colour vision and can see red 

What is moth pollination?

Phalaenophily e.g. gardenia

• Flowers are white (stand out at night) 

• Nectar is kept in deep corolla tubes 

• More nectar is produced because moths have higher energy requirements 

• Strong scent 

What is bird pollination?

Ornithophily e.g. hibiscus

• Flowers are brush or tube shaped, and the nectar is secreted into short, wide, spurs 

• Flowers are tough (beak is stronger than tongue) 

• Lots of nectar is produced 

• Flowers are always red 

What is bat pollination?

Chiropterophily e.g. cacti

• Bats are colour blind but can smell strongly 

• Flowers are not within the foliage, but hang below for easy access 

What is deceit pollination and when is it useful?

This is when plants take advantage of animals without giving them actual rewards - it saves energy but is only useful when the mimic flowers are at low frequencies within the community 

What’s an example of deceit pollination?

Orchids of the genus Ophrys produce flowers that mimic the female species of an insect and because this mimicry releases instinctive behaviours in an animal, it cannot learn to avoid these flowers

What is sapromyophily?

These flowers attract insects that feeds or lay eggs on rotting protein by looking dead and smelling badly 

What is a flower?

The determinate bisexual reproductive structure of an angiosperm

What are the two angiosperm species that we have studied, and why have we studied them?

Antirrhinum and Arabidopsis (they are easy to grow in a greenhouse, they are easy to cross-pollinate, their flowers are easy to see, and they grow quickly)

What is the ABC model?

A flower has four whorls, whose development is determined by three genes 

• Whorl 1 – A genes – sepals develop 

• Whorl 2 – A and B genes – petals develop 

• Whorl 3 – B and C genes – stamens develop 

• Whorl 4 – C genes – carpels develop 

(limits of zone A and C are set by mutual interaction - they repress each other) 

What is the function of the A genes?

Antirrhinum – ovulata, Arabidopsis – apetala2 
^They encode transcription factors

What is the function of the B genes?

Antirrhinum – deficiens and globosa, Arabidopsis – apetala3 and pistillata
^These encode MADS box transcription factors which only work when paired together as two heterodimers (if one protein is missing, the pair can’t form, so mutants missing either gene show the same traits) 

What are transcription factors?

Proteins, encoded by genes, which control the expression of other genes 

What do transcription factors do?

They bind to a gene’s promoter region (a special DNA region where RNA polymerase attaches to start making messenger RNA) 

They have two parts – one that sticks to the DNA and one that activates the gene 

Transcription factors are grouped into families based on…

…the structures of their DNA binding domains e.g. MADS box transcription factors

What is the function of the C genes?

Antirrhinum – plena, Arabidopsis – agamous 
^Both genes encode MADS box transcription factors

How can we describe the ABC model in ferns, mosses, and liverworts (non-flowering land plants)?

They don’t have homologues of the ABC genes (even though they have MADS box genes) 
^This suggests that the origin of the ABC genes likely evolved after these groups diverged, coinciding with the origin of flowers in angiosperms

How can we describe the ABC model in gymnosperms?

B and C function genes are present in conifers and cycads (gymnosperms) 

• Gymnosperms don’t produce flowers but have separate male cones (pollen) and female cones (ovules) 

• B function genes are found only in male cones, C function genes are found in both male and female cones (expressing both B and C will lead to the development of male cones, expressing only C will lead to the development of female cones) 

• B and C genes originally evolved to specify male and female reproductive organs before the evolution of flowers

How can we describe the ABC model in monocots?

• Monocots have two whorls of colourful tepals instead of sepals and petals 
  ^This happens because B function genes are expressed in whorls 1, 2, and 3, causing tepals to form in the first two whorls 

• In tulip B mutants, whorls 1 and 2 develop sepals and whorl 3 develops carpels, matching ABC model predictions (B function expression has changed in monocots) 

• This shows that the BC part of the ABC model is ancient in seed plants (was in the common ancestor) 

What is gene duplication?

The process where a segment of DNA containing a gene is copied, resulting in two or more copies of that gene in the genome (caused by unequal crossing over, transposition, polyploidy) - much of the variation in angiosperms is caused by gene duplication

What is unequal crossing over?

This happens during meiosis when chromosomes misalign, causing one chromosome to gain extra DNA (a duplication) and the other to lose it (a deletion) - this process is most common in repetitive regions

What are the results of gene duplication?

• Pseudogene – one a gene has been duplicated, it may not fill any function 

• Subfunctionalisation – the two duplicated might split the roles of the original gene 

• Neofunctionalisation – the new copy of the gene may be recruited to new roles, because the original copy is still there 

What is transposition?

This is where a chunk of DNA is moved from one chromosome to another because it is attached to a transposon - this can be beneficial and harmful, depending on the importance of the DNA 

What is polyploidy?

The condition of having more than two complete sets of chromosomes in an organism, and because many of these genes are duplicates, the plant doesn’t need them all right away - so over time, some can change and take on new roles, leading to new functions and lots of variation 

Major changes in angiosperms could be the results of…

…duplication and adaptation of transcription factors

What are MADS box genes?

• Their two types (type 1 and type 2) are present in plants and animals, so at least one ancestral MADS box gene was duplicated in the common ancestor

• Some MADS box genes are expressed in roots or embryos, and phylogenetic studies suggest that these belong to ancient branches of the gene families 
  ^Earliest MADS box genes in plants likely helped build basic structures like leaves, roots, and stems, later, after gene duplications, some of these genes were adapted to control flower development 

• MADS box genes in the ABC genes have arisen through duplication events at the time the first land plants appeared

What is the MYB transcription factor family?

• MYBs existed before animals and plants diverged and have greatly expanded in plants through duplication 

• They have a special DNA binding region – the MYB repeat 

• The original MYB had 3 repeats (R1, R2, R3), but most plant MYBs have only 2 (R2 and R3), while some have all 3 or just 1 
  ^This suggests that during evolution, the MYB gene family underwent partial duplications and deletions, leading to diverse versions of the gene with different numbers of repeats 

What are the roles of 3 repeats, 2 repeats, and 1 repeat in the MYB repeat?

• 1 repeat MYBs – their subgroup will recognise and bind to the telomeres of chromosomes (conserved, all genes do the same thing)

• 2 repeat MYBs – these genes have many different functions (sdiversified through subfunctionalisation and neofunctionalisation) 

• 3 repeat MYBs – involved in control of the cell cycle (conserved) 

How might some MYBs reduce the level of expression of certain genes?

Either by directly repressing transcription, or by competition with other transcription factors which might be stronger activators 

R2R3 MYBs seem to be involved in…

…processes specific to plants e.g. the control of secondary metabolism 

What is secondary metabolism?

The production of compounds that are not absolutely essential for growth and survival of the plant

How has secondary metabolism evolved convergently?

Secondary metabolism can evolve convergently in different species e.g. some plants can release HCN as a defence compound 
^This shows that gene duplication can lead to both diversity and similarity (convergence) in plant traits 

How we can we describe maize and teosinte?

Maize – tall, unbranched with one main stalk that produces many ears of corn from female flower clusters, it has separate male and female flowers on the same plant, with males at the top and females above the leaves 

Teosinte – many branches and fewer female flower clusters 

Maize and teosinte can interbreed (and produce fertile offspring) and are classified as subspecies 

What are the differences between maize and teosinte due to?

The differences between them come down to changes in just one gene - the teosinte branched gene 

• This gene encodes a transcription factor that controls side branching

• In teosinte, it only turns on in shade to stop branching and save energy 

• In maize, it’s always on, so maize grows with just one main stalk 

• If maize has a mutation that turns this gene off, it grows like teosinte in the sun, with lots of branches

What is the evidence that floral traits are under strong selective pressure?

Flower colour in wild radish is under selective pressure from butterflies 

• Wild radish can be white or yellow (a single locus controls flower colour, with two alleles at it) 

• Bees will collect nectar from either colour flower 

• The cabbage white butterfly prefers yellow flowers over white flowers 

• Selective pressures can be pollinator-specific 

Flower colour in Delphinium is under selective pressure from bees and hummingbirds 

• Delphinium flowers can be blue or white 

• Pollinators (hummingbirds and bumblebees) prefer to visit blue flowers because extracting nectar is less effortful 

• Two different sorts of pollinators are applying strong selective pressure on flower colour (the blue flower allele is always selected for) 

Petal cell shape (controlled by MYB transcription factor) is under selective pressure from bees 

• The mixta mutant of Antirrhinum doesn’t have cone shaped epidermal cells and is slightly paler in colour (the MIXTA gene encodes a MYB transcription factor) 

• Bees favour flowers with cone shaped petal cells (because they help the bees to grip the flower) 

• Bees are imposing selective pressure to maintain the wild type allele in the population 

Too much herbivory causes a shift in flowering time of Nictotiana attenuata 

• Nicotiana attenuata flowers open at night and attract hawkmoths (which lay eggs and hatch into caterpillars) 

• When herbivory levels get too high, Nicotiana attenuata plants switch the time of flower opening to morning and attract hummingbird pollinators instead 

Selection and the evolution of herbicide resistance in weeds 

• This is a very strong selective pressure which favours any plant that has acquired the ability to survive the herbicide treatment 

How might plants become less susceptible to herbicides?

Enhancing their metabolism

• This detoxifies herbicides before they act

• E.g. in cereal plants, two types of enzymes (cytochrome P450 monooxygenase and glutathione S-transferase) are responsible for the detoxification of herbicides

Target site resistance

• Site of herbicide activity is blocked so that the plant doesn’t experience any poisoning (absolute resistance, but usually only in group of herbicides) 

• E.g. a mutation in the gene encoding acetyl co-enzyme A carboxylase (ACCase) will inhibit the herbicide fenoxaprop from inhibiting the enzyme and inhibiting the catalysis of the first step of fatty acid synthesis 

What can farmers do regarding herbicide resistance?

Farmers can stop herbicide resistance by evolving to make sure there is no selective pressure favouring it, and they can keep selective pressure under control by using non-herbicide-based control methods 

How can GM crops affect wild crops?

• Seeds from GM plants can escape the harvesting process and be scattered in nearby fields (this is only dangerous if the GM plants have sufficient selection acting to give them an advantage over wild plants) 

• The pollen of the GM plants will pollinate nearby wild plants - this can form a hybrid plant (it is unlikely that these will outcompete the wild species)